The rheological behaviour of concentrated aqueous dispersions of graphene oxide (GO) was studied as a model system and then compared to those of GO in poly(methyl methacrylate) (PMMA). Dynamic and steady shear tests were conducted using a parallel plate rheometer. The aqueous system behaved as a reversibly flocculated dispersion with linear viscoelastic regions (LVR) extending up to strains of 10 %. Dynamic frequency sweeps conducted within the LVR showed a classic strong-gel spectrum for high concentrations. Under steady shear, the dispersions shear-thinned up to a Peclet number (Pe) < 1, followed by a power law at higher Pe. The dispersions were thixotropic and recovered their structure after 60 min rest. The change in rheological properties of the PMMA upon the addition of the GO was less pronounced possibly due to the absence of hydrogen bonding; a relatively small increase in viscosity was found, which is encouraging for the melt processing of graphene composites.